The acquisition of data and subsequent generation of computer models for real-world objects is of interest in many industries and for many applications including architecture, physical plant design, surveying, manufacturing quality control, medical imaging and construction, as well as cartography and geography applications. In order to obtain accurate coordinates or 3D models of an object, as well as the area in which that object exists in the real world, it is necessary to take accurate measurements, or samplings of surfaces that make up the object, and elements of the surrounding area. Historically, this sampling was carried out using techniques that provided samples at the rate of tens or hundreds per hour at most.
Recent advances in scanning technology, such as technologies utilizing LIDAR scanning, have resulted in the ability to collect billions of point samples on physical surfaces, over large areas, in a matter of hours, minutes or even seconds. In a scanning process, the scanning device scans a laser beam across a scene that encompasses the structure of interest and the beam reflected from the scene is captured by the scanning device. The scanning device thus measures a large number of points that lie on surfaces visible in the scene. Each scan point has a measured location in 3D space, within some measurement error, that typically is recorded relative to a point (x,y,z) in the local coordinate system of the scanner. The resulting collection of points is typically referred to as one or more point clouds, where each point cloud can include points that lie on many different surfaces in the scanned view.
Conventional scanning systems do not natively create points but instead create sets of ranges with associated mirror angles which are converted to x, y and z coordinates. The function which maps these native measurements into x, y and z coordinates depends on how the scanner was assembled and, for high accuracy systems, is different for each scanner and is a function of temperature and other environmental conditions. The differences between scanner systems are typically represented by a collection of numbers called calibration parameters.
For acquisition of a precise position of targets, in particular in field of surveying, total stations are mainly used today. Such targets could be typified by non-reflective or reflective objects like prisms. A total station is a manually operated optical instrument used in surveying. A total stations is a combination of an electronic theodolite (transit), an electronic distance meter (EDM) and software running on a computer e.g. a remote computer known as data collector. With a total station one may determine angles and distances from the instrument to points to be surveyed. With the aid of trigonometry and triangulation, the angles and distances may be used to calculate the coordinates of actual positions (x, y, and z or northing, easting and height) of surveyed points, or the position of the instrument from known points, in absolute terms. Modern total station instruments measure angles by means of electro-optical scanning of extremely precise digital bar-codes e.g. etched on rotating glass cylinders or discs within the instrument.
As the aforementioned systems work with laser light and enable measuring of object within relative great distances the hazardousness of human and animal tissue, in particular concerning eyes of human beings, to be damaged is quite high when entering an area where such a measurement is performed.
With view to laser safety, DE 10 2005 027 208 describes a method for controlling a laser scanner in a way that laser power is reduced or the laser is switched off depending on a distance to an object. Furthermore, it is proposed to define a fixed laser power during a scan. The scan is started with maximum laser output power. If the minimum distance measured is smaller than the processed safe distance the scan is stopped, the laser output power is scaled down and the scan is started again. This process is recursive. If the measured distance is smaller than the minimum safe distance at a defined minimum laser output and being the lowest laser output power step, the scan is canceled.
Additionally, US 2005/0205755 proposes a method for adaptive controlling laser parameters in dependency on environmental influences. In that context, in near field the laser output power is reduced to accommodate the laser safety limits. According to an embodiment, for maintaining safety regulations two distance ranges are used—one with limited laser output power and another with full laser power. The laser power amplitude is controllable by an optical attenuator, e.g. filter wheel, from 20 up to 200 amplitude steps. The attenuator is set depending on range or receiving signal strength.
According to the technical solution of DE 10 2005 027 208 the laser power is fixed while the scan is running and, thus, a disadvantageousness of varying measurement performance over the scanned area caused by different distances to be measured remains.
According to the technical solution of US 2005/0205755 the laser power, in particular the amplitude, is adjustable for meeting laser safety as well, but still the problem of varying measurement performance within one scanning period remains. In particular a signal-to-noise ratio of the systems is not considered for scanning, wherein this ratio influences the quality of measurement and is depending on the distance to an object and, therefore, changes with distance.